A new risk model promises to enhance safety evaluations of bridges in regions prone to natural disaster by providing for the assessment of multiple hazards.

Dr. Swagata Banerjee Basu

Swagata Banerjee, an assistant professor of civil engineering at Penn State University, has developed a new hazard evaluation framework which is capable of accounting for the impact of both seismic and flood dangers on bridges in areas which are susceptible to both.

Her new method marks a major advance upon preceding risk evaluation models which only address the effects of a single hazard category. It also provides far more realistic assessments of bridge safety, given that so many parts of the world are afflicted by multiple forms of natural hazard.

The prime example of this is the combination of flood-induced scour, which can reduce the structural resilience of bridges, with seismic events. Once the earth and rock around the foundations of a bridge is eroded by flooding, the bridge itself becomes far more vulnerable to damage in the case of earthquake.

For bridges which are subject to these conditions, the ideal assessment model would account for both of these factors instead of just focusing on one or the other.

Banerjee's risk evaluation process attempts to address this issue, by first using design drawings of existing bridges as the basis for the construction of computer models. Historical flood data for the bridges is used to create a flood hazard curve in order to determine the peak local discharge in the event of major flooding. This in turns permits the calculation of the scour depth at the bridge's foundations.

Once the impact of flooding and erosion on a bridge is complete, seismic analysis is then performed based on a model of the now-compromised structure.

Banerjee has already used the technique to provide evaluations of two concrete bridges in California based on designs provided by the California Department of Transportation - one which commenced operation in 2009, and another which was constructed several decades ago. Her analysis found that the seismic performance of the older bridge would be significantly compromised by flood events.

The new method could prove highly applicable to those large swathes of the United States which are both earthquake-prone and vulnerable to flooding, such as California, Washington state, as well as several states within the New Madrid seismic zone, including Missouri, Tennessee and Arkansas.